Battery Monitoring Basics

October 2012 Dallas 1

Battery Monitoring Basics

2October 2012 Dallas 2

Section 1 – Basic Concepts

• What does a battery monitor do?

• How to estimate battery capacity?

– Voltage lookup

– Current integration

• Factors affecting capacity estimation

• Other functions

– Safety and protection

– Cell balancing

– Charging support

– Communication and display

– Logging


What does a battery monitor do?

• Capacity estimation

• Safety/protection• Charging support• Communication

and Display• Logging• Authentication

Batte

ryGasGauge

RsIbatt

Vbatt

Load

Cha

rger

VCHG

ICHG

comm

CHG DSG

IDSG

VDSG

VPACK

CellMonitorSystem

Tbatt

Battery Subsystem


How to estimate battery capacity?

• Measure change in capacity– Voltage lookup– Coulomb counting

• Develop a cell model– Circuit model– Table Lookup


Voltage lookup

)(tq

mLmarks

V(t)

I(t)

• One can tell how much water is in a glass by reading the water level– Accurate water level reading

should only be made after the water settles (no ripple, etc)

• One can tell how much charge is in a battery by reading well-rested cell voltage– Accurate voltage should only be

made after the battery is well rested (stops charging or discharging)


OCV curve

Level rises same rate

Level rises same rate

Vo

lta

ge

Fullness

OCV Curve

Full charge voltage

End of discharge voltage

0% 100%Capacitor

Level risesfaster

Level risesslower

Vo

lta

ge

Fullness

OCV Curve

Full charge voltage

End of discharge voltage

0% 100%Battery


OCV voltage table: DOD representation

OCV(DOD)

2900

3100

3300

3500

3700

3900

4100

4300

0 0.2 0.4 0.6 0.8 1 1.2

DOD

Vo

ltag

e_a(

DO

D)

Voltage_a

Poly_a(DOD)

Flat Zone

VmaxVmin

DOD = Depth of DischargeSOC = State of ChargeDOD = 100% - SOC


Current integration

• One can also measure how much water goes in and out

• In batteries, battery capacity changes can be monitored by tracking the amount of electrical charges going in/out

• But how do you know the amount of charge, , already in the battery at the start?

• How do you count charges accurately?

)(tq

mLmarks

Voltage

I(t) dttIqtq )()( 0

k kk Itqq 0

0q


Basic Smart Battery System

Batte

ry M

odel

GasGauge

RsIbatt

Vbatt

Load

Cha

rger

VCHG

ICHG

comm

CHG DSG

IDSG

VDSG

VPACK

Tbatt

k kk Itqq 0


Circuit model

• VOC a function of SOC• Rint is internal resistance• Rs and Cs model the short

term transient response• RL and CL model the long term

transient response• Vbatt and Ibatt are the battery

voltage and current• All parameters are function of

temperature and battery age

Voc(SOC)

Rint RL

CS CL

RS Vbatt

Ibatt

DC model

Voc(SOC)

Rint RL

CS CL

RS Vbatt

Ibatt

Transient model


Table lookup

• Large, multi-dimensional table relating capacity to– Voltage– Current– Temperature– Aging

• No cell model• Apply linear interpolation to make lookup “continuous”• Memory intensive


Factors affecting capacity estimation

• PCB component accuracy• Instrumentation accuracy• Cell model fidelity• Aging• Temperature


PCB component accuracy

• Example– Current sensing resistor– Trace length (resistance)

sRtItV )()(

GasGauge

Rs

)(tI

rsR

tVtI

)()(

R+ R-


Instrumentation accuracy

ADC count

Vol

tage

• ADC Resolution• Sampling rate• Voltage drift / calibration• Noisy immunity


Battery model fidelity

• Steady-state (DC)• Transient (AC)• Capacity degradation

– Aging– Overcharge

Voc(SOC)

Rint RL

CS CL

RS Vbatt

Ibatt

Transient model

Voc(SOC)

Rint RL

CS CL

RS Vbatt

Ibatt

DC model


Model parameter extraction

• Extract battery model parameter values using actual collected battery data– Open circuit voltage (OCV)– Transient parameters (RC)– DC parameters (Ri)

• Least square minimization• Extraction process can be hard and time consuming


Temperature

• Temperature is important for– Capacity estimation– Safety– Charging control

• Temperature impacts model parameters– Resistance– Capacitance– OCV– Max capacity


Safety

• High operating temperature

– Accelerates cell degradation

– Thermal runaway and explosion

• LiCoO2 – Cathode reacts with

electrolyte at 175°C with 4.3 V

• Cathode coatings help considerably

• LiFePO4 shows huge improvement!

Thermal runaway is > 350°C

OCV = 4.3 V

100 125 150 175 200 225 250Temperature (°C)

Hea

t Flo

w (W

/g)

Thermal Runaway


Cell Safety

Safety Elements• Pressure relief valve

• PTC element

• Aluminum or steel case

• Polyolefin separator

– Low melting point (135 to 165°C)

– Porosity is lost as melting point is approached

– Stops Li-Ion flow and shuts down the cell

• Recent incidents traced to metal particles that pollutes the cells and creates microshorts


Safety and protection

• Short circuit• Over/under

(charge/discharge) current

• Over/under voltage• Over temperature• FET failure• Fuse failure• Communication failure• Lock-up• Flash failure• ESD• Cell imbalance

Alert Trip

Trip Margin(time)

time

TripMargin(level)

TripLevel

Trip-OverTrip

Alert Trip

Trip Margin(time)

time

TripMargin(level)

TripLevel

Trip-UnderTrip


Overcurrent Protection Details

Gas-Gauge ICSoftware Control

Both CHG and DSG(1-s Update Interval)

AFEHardware Protection

AFEOCP (DSG Only)

Turn Off FETs

AFESCP (CHG and DSG)

Turn Off FETs

BatteryCurrent

Recoverable

Recoverable

Time

1st-Level OCP(1st Tier)

Turn Off FETs

1st-Level OCP(2nd Tier)

Turn Off FETs

2nd- evel Safety OCP(Blow Chemical Fuse)

L

Recoverable

AFE OCP

DSG Tim e

1 to 31 m s ~

AFE SCP CHG/DSG Time

0 to ~915 µs

Safety OCP CHG/

1 60 sDSG Tim e

to ~

OCP (2nd Tier)CHG/DSG Tim e

1 to ~60 s

OCP (1st Tier)CHG/DSG Tim e

1 to ~60 s

Permanent

Recoverable


Basic Battery-Pack Electronics

• Measurement: Current, voltage, and temperature• bq20zxx gas gauge : Remaining capacity, run time, health condition• Analog front end (AFE)

Discharge MOSFET Charge MOSFET

Temp Sensing

Pack+

Pack–

Voltage ADC

OvervoltageUndervoltage

Gas Gauge IC

I2C

Chemical Fuse Q2Q1

SenseResistor

bq20z90

Second SafetyOVP IC

bq29412

AFE

RT

Current ADC Rs

SMBusSMDSMC

bq29330

OCPCell

Balancing

LDO


JEITA/BAJ Guidelines for Notebook

• Do not charge if T< 0°C or T> 50°C• Minimize temperature variation among cells• How do we collect temperature information?

4.15 V4.20 V

Upper-Limit Voltage: 4.25 V

T1 T2 T5 T6 T3 T4

Safe Region

(100C) (450C)

No C

harge

Upper-Limit Charge Current

No C

harge


Why Are Battery Packs Still Failing?

• Space-limited design causes local heat imbalance

• Cell degradation accelerated

• Leads to cell imbalance

• Single/insufficient thermal sensor(s) compromise safety

Temperature Profile along Section Line

>10ºC Variation Between

Cells

→ Heat Imbalance


25

Cell Balancing

Battery cells voltages can get out of balance, which could lead to over charge at a cell even though the overall pack voltage is acceptable.

Cell balance can be achieved through current bypass or cross-cell charge pumping


Passive Cell Balancing: Simplest Form

• Simple, voltage based• Stops charging when

any cell hits VOV threshold

• Resistive bypassing turns on

• Charge resumes when cell A voltage drops to safe threshold

VOV

Cell A

Cell BCell B

ta tb tc td te tf

VDiff_End

V – VOV OVH

VDiff_Start

bq77PL900, 5 to 10 series-cell Li-Ion battery-pack protector for power tools

+

BatteryCell

Ibalance

Rext1

Rext2


R4

Q2

R4

Q2

Fast Passive Cell Balancing

• Needed for high-power packs, where cell self-discharge overpowers internal balancing

• Fast cell balancing strength is 10x ~ 20x higher

Cell 2

PACK +

1 k

1 k

R2

R1

bq2084/bq20zxx

Cell 1

R

1 k

R3

CellCB

DS(on)

VI

RInternal CB

CellCB

VI

R4Fast CB

RDS(on)

Where R4 << RDS(on)


Charging support

• Inform battery charger proper charging voltage and current

• Conform to specification (e.g., JEITA)• Reduce charge time• Extend battery life by:

– Avoid overcharging– Precharging depleted and deeply discharged cells


Communication and Display

• Communication– To the System or Charger– Industry specification

• Display– LED, LCD– Capacity indication– Fault indication


Logging

• Works like an airplane “blackbox recorder”

• Record important lifetime information– Max/min voltage– Max/min current– Max/min temperature

• Record important data for failure analysis– Reset count– Cycle count– Excessive flash wear


Section 2

Battery Fuel Gauging:CEDV & Z-track


Basic Vocabulary Review

• Capacity– Design Capacity [mAh]– Qmax, Chemical Capacity [mAh]– FCC, Usable Capacity [mAh]– RM, Remaining Capacity [mAh]– RSOC [%]– DOD [%]– DOD0, DOD1 [%]

• Voltages– OCV [mV]– OCV(DOD) [mV]– EDV [mV]– EDV 2 [mV]– EDV 0 [mV]– CEDV [mV]

• Current– C-rate [mA]– Coulomb Counting

dttIqtq )()( 0


• External battery voltage (blue curve) V = V0CV – I • RBAT

• Higher C-rate EDV is reached earlier (higher I • RBAT)

EDV

Full chemical capacity: Qmax

Usable capacity : FCC

0 1 2 3 4 6

3.0

3.5

4.0

4.5

Capacity, Ah

Voltage, V

How Much Capacity is Really Available?

Open circuit voltage (OCV)

I • RBAT

04/19/23 34October 2012 Dallas 34

What Does A Fuel Gauge Do?

3V

4.2V

Which route is the battery taking?

Suppose we are here

0%

• What is the remaining capacity at current load?

• What is the State of charge (SOC)?

• How long can the battery run?

04/19/23 35October 2012 Dallas 35

Current Integration Based Fuel-gauging

• Battery is fully charged• During discharge capacity

is integrated • State of charge (SOC) at

each moment is RM/FCC• FCC is updated every

time full discharge occurs

0%3V RM = FCC - Q

SOC = RM/FCC

4.2V

Q

FCC

04/19/23 36October 2012 Dallas 36

Learning Before Fully Discharged – fixed voltage thresholds

7%3%

0%EDV0

FCC

4.2V

• It is too late to learn when 0% capacity is reached Learning FCC before 0%

• We can set voltage threshold that correspond to given percentage of remaining capacity

• However, true voltage corresponding to 7% depends on current and temperature

EDV2

EDV1

04/19/23 37October 2012 Dallas 37

Learning before fully discharged with current and temperature compensation

4.2V

EDV2 (I1)

EDV2 (I2)

OCV

• Modeling last part of discharge allows to calculate function V(SOC, I, T)

• Substituting SOC=7% allows to calculate in real time CEDV2 threshold that corresponds to 7% capacity at any current and temperature

CEDV Model:Predict V(SOC) under any current and temperature

CEDV


CEDV Model Visualization

3% 4% 5% 6% 7% 8% 9%

Actual battery voltage curve

VoltageOCV curve defined

by EMF, C0

OCV corrected by I*R (R is defined by

R0, R1, T0)I*R

Further correction by low temperature (TC)

Reserve Cap: C1 shifts fit curve laterallyBattery Low


39

CEDV Formula

CEDV = CV - I*[EDVR0/4096]*[1 + EDVR1*Cact/16384]*[1 – EDVT0*(10T - 10Tadj)/(256*65536)]*[1+(CC*EDVA0)/(4*65536)] * age

Where:CV = EMF*[1 – EDVC0*(10T)*log(Cact)/(256*65536)]Cact = 256/(2.56*RSOC + EDVC1) – 1 for (2.56*RSOC + EDVC1) > 0Cact = 255 for (2.56*RSOC + EDVC1) = 0EDVC1 = 2.56 * Residual Capacity (%) + “Curve Fit” factorTadj = EDVTC*(296-T) for T< 296oK and Tadj < TTadj = 0 for T > 296 oK and Tadj max value = Tage = 1 + 8 * CycleCount * A0 / 65536.


Impedance Track Fuel Gauging

• Combine advantages of voltage correlation and coulomb counting methods

• State of charge (SOC) update:– Read fully relaxed voltage to determine initial SOC and capacity

decay due to self-discharge– Use current integration when under load

• Parameters learning on-the-fly:– Learn impedance during discharge– Learn total capacity Qmax without full charge or discharge– Adapt to spiky loads (delta voltage)

• Usable capacity learning:– Calculate remaining run-time at typical load by simulating voltage

profile do not have to pass 7% knee point


Current Direction Thresholds and Delays

Example of the Algorithm Operation Mode Changes With Varying SBS.Current( )

1

2

3

4

5

6

7

1. CHG relaxation timed2. Enter RELAX mode3. Start discharging4. Enter DSG mode5. DSG relaxation timed6. Enter RELAX mode7. Start charging8. Enter CHG mode

8


What is Impedance Track?

1. Chemistry table in Data Flash:

OCV = f (dod)

dod = g (OCV)

2. Impedance learning during discharge:

R = OCV – V

I

3. Update Max Chemical Capacity for each cell

Qmax = PassedCharge / (SOC1 – SOC2)

4. Temperature modeling allows for temperature-compensated impedance to be used in calculating remaining capacity and FCC

5. Run periodic simulation to predict Remaining and Full Capacity

10,000 foot View


Close OCV profile for the Same Base-Electrode Chemistry

• OCV profiles close for all tested manufacturers

• Most voltage deviations from average are below 5mV

• Average DOD prediction error based on average voltage/DOD dependence is below 1.5%

• Same OCV database can be used with batteries produced by different manufacturers as long as base chemistry is same

• Generic database allows significant simplification of fuel-gauge implementation at user side

0 0.25 0.5 0.75 14

2.67

1.33

0

1.33

2.67

4

Manufacturer ABCDE

DOD, fraction

DOD

error,

%

0 0.25 0.5 0.75 10.015

0.01

0.005

0

0.005

0.01

0.015

Manufacturer ABCDE

DOD, fraction

Devia

tion,

V

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 13.4

3.67

3.93

4.2

Manufacturer ABCDE

DOD, fraction

Volta

ge, V


Resistance Update

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

100

200

300

400

dod

Ra

Before Update

Discharge direction


Ra Table: Interpolation and Scaling Operation

• R = (OCV – V) / Avg Current. Averaging method is selectable

• Resistance updates require updating 15 values for each cell

• A new resistance measurement represents the resistance at an exact grid point. Exact value found by interpolation

• All 15 grid points are ratiometrically updated from any valid gridpoint measurement. Changes are weighted according to confidence in accuracy

Gri

d 0

Gri

d 1

4

k:

Pre

sen

t g

rid

m:

Last

vis

ited

gri

d

Ra_newRa_old

Step 1

Step 2

Step 3

Interpolation

Scale “After”

Scale “Before”


Timing of Qmax Update


FCC Learning

0 0.2 0.4 0.6 0.87200

7400

7600

7800

8000

8000

9000

1 104

1.1 104

1.2 104

1.3 104

SMB FCCtrue FCC

Ra gridsVoltage

SMB FCCtrue FCC

Ra gridsVoltage

DOD

FC

C, m

Ah

V, m

V


48

Modeling temperature

Voc(Vsoc)

R i

C

R

Vbatt

Ibatt

acibattocbattibattp TTAhRIVVR

RIdt

Tdcm 22 1

m := cell masscp := specific heat

hc := heat transfer coefA := cell surface area

Ta := ambient temp

Heating Cooling

• Based on a heating / cooling model **• Heating is from the internal resistance• Cooling is from heat transfer to the

environment, i.e.,• How many thermistors?

** “Dynamic Lithium-Ion Battery Model for System Simulation”, L. Gao, S. Liu and R. A. Dougal, IEEE Transaction on Components and PackagingTechnologies, vol. 25, no. 3, September 2002.

aTT


RemCap Simulation (concept)

Constant Load Example

I

Qstart ΔQ ΔQ ΔQ

ΔQ/2

ΔQ/4

. . . . . RsvCap

Vterm

Δ V > 250mV

EDV

V

(loaded)

Start of discharge

RemCap

Time

Time

OCVI*R


Z-track Accuracy in Battery Cycling Test

0 50 100 150 200 250 3001.5

1

0.5

0

0.5

1

error at 10%error at 5%error at 3%

Cycle Number

Rem

aini

ng C

apac

ity

Err

or,

%

• Error is shown at 10%, 5% and 3% points of discharge curve

• For all 3 cases, error stays below 1% during entire 250 cycles

• It can be seen that error somewhat decreases from 10 to 3% due to adaptive nature of IT algorithm


Property CEDV Impedance Track

Worst error new, learned +/-2% +/-1%

Worst error aged, learned +30% (+/- 15% with age data)

+/-2%

Data collection 3 temperatures, 2 rates,

Fitting to obtain parameters.

2 weeks

Chemistry selection test,

Optimization cycle

1 week

Instruction flash small large

Voltage accuracy requirement

20mV/pack 3mV/pack

State of charge initialization (host side requirement)

No Yes

FCC temperature compensation

No (with rare exceptions) Yes

FCC rate compensation No (with rare exceptions) Yes

Learning cycle in production required Not required

CEDV, Impedance Track Comparison


Documents

Battery Monitoring Basics